This invention relates to the field of protective coatings for use on metallic parts which are used at elevated temperatures. This invention has particular utility in the field of gas turbine engines. It is conventional to use coatings on almost all gas turbine parts which encounter severe operating conditions at elevated temperatures. These parts include the burner assembly, turbine vanes and blades.
Perhaps the most advanced coatings now in use in gas turbine engines are those which are termed MCrAlY overlay coatings where M is a metal chosen from the group consisting of iron, nickel, cobalt and mixtures of nickel and cobalt, Cr is chromium, Al is aluminum and Y is yttrium or equivalent reactive metal. Typical of these MCrAlY coating alloys are those described in U.S. Pat. Nos. 3,542,530, 3,676,085, 3,754,903 and 3,928,026 which are all assigned to the assignee of the present invention. U.S. Pat. No. 3,918,139 which is also assigned to the present assignee describes an MCrAlY overlay coating which contains an addition of from three to twelve weight percent platinum or rhodium.
In the coating art, attempts have been made to employ platinum as a protective coating or a part of a protective coating system for gas turbine parts. U.S. Pat. No. 3,819,338 to Bungardt et al. suggests the use of platinum in a diffusion coating. According to this patent a layer of platinum is applied to the part to be protected followed by a conventional diffusion aluminizing treatment. In an alternative embodiment the platinum and aluminum are deposited simultaneously. A similar teaching is found in U.S. Pat. No. 3,961,910 to Baladjanian et al. In this patent the metal used is rhodium and a thin layer of rhodium is applied to the part to be protected followed by a conventional diffusion aluminzing coating. The rhodium layer is suggested to minimize the diffusion between the aluminide layer and the substrate which is being protected. U.S. Pat. No. 4,070,507 by Stueber et al. contains yet another teaching of the use of a platinum-rhodium layer prior to diffusion aluminiding. In this patent, a first coating of rhodium is applied followed by a second coating of platinum followed thereafter by a diffusion aluminizing treatment.
U.S. Pat. Nos. 3,976,436 and 4,018,569 (a division of U.S. Pat. No. 3,976,436) by Chang describe alloys and coatings based on MCrAlY coatings containing in addition from 0.1 to 10% hafnium and 0.5-20% of an element selected from the group consisting of platinum, rhodium and palladium. Suggested application techniques include diffusional coating techniques in combination with aluminiding. U.S. Pat. Nos. 4,123,594 and 4,123,595 both by Chang also relate to platinum containing protective coatings. These patents both describe coating systems including an inner graded coating which contains chromium, aluminum, hafnium and up to 30% platinum. An outer coating contains from 10 to 50% aluminum and 1 to 40% hafnium, platinum, rhodium or palladium in the case of U.S. Pat. No. 4,123,594 and 5 to 50% hafnium, platinum, rhodium, palladium in the case of U.S. Pat. No. 4,123,595. The method of coating application is a complex on in which after application of the first coating portion by vapor deposition the coated article is treated to cause substantial diffusion of the coating with the substrate after which the outer coating layer is applied through a process such as sputtering. In U.S. Pat. No. 4,123,594 the outer coating may also be applied by a pack deposition process.